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Manual J is wrong for the 1st time???

As my post stated, I have ran across a manual j that is showing me the wrong size system for the 1st time. I know what you are thinking the input details have to be wrong but I have triple checked them.

Now let me explain. I have a customer with a 1800 square foot home that was built in the 50's which is rasied frame home with no insulation in attic or the floor. The walls are more then likely not insulated as well. The house can't be much tighter then average.

He has been running a 4 ton system in the home for 10 plus years with no problem keeping it 72 on a 95 degree day. Some more info on the home has a good bit of windows all single wood with metal storms on them.

My load came out to be 6.5 tons of air needed to cool the home due to no insulation but this man home has done fine for years with his existing (4) ton system as we have been doing all his work for the past 5 years.

Other notes the system is even mismatched using a goodman a/c unit with a tempstar coil. The system has done the job. So Made a call to install another (4) ton system rated at 48,000 btus and we have installed it and per customer it is doing good. The duct system would only handle a 4 tons of air anyways along with returning it.

Has anyone one else ever had this happen to them where rule of thumb has worked for them over manual j? I hate even saying that as I belive strongly in manual j but this time it was wrong! Any input to help me understand this would be great...

Yeah, I have gotten some comments from contractors and architects that load calculation numbers have been over estimated at times. I assume there are just factors that aren't accounted for... like shading from adjacent buildings, trees, or just curtains.

Heck, maybe the asbestos material in the plaster give added insulation properties IDK... If the house is on a slab, then there are thermal mass properties... and there is a factor of safety in all the equations. If the 4 ton system works, then go with the 4 ton. 6.5 tons will be 2 systems in residential anyways.

You can call me Sam

It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

I just sold a system to my friend with a similar house...built in 50's with the returns on the outside wall and supplies on inside wall...minimal insulation,
1200 sq ft on first floor with (6) 6" heat runs and a 600 sq ft loft with (2) 6" runs.

My supplier and I came up with about 2.5 Tons with about 72,000 btu heating ( we are talking Buffalo so we have to have a good heat load).

I will be reducing the bathroom to 4" and upping the 2nd floor to 7" runs.

4 tons sounds like a a lot for a 1800 sq ft house unless your in an area with a large year round cooling load. you would have to have about (16) 6" runs to accomodate that CFM capacity with a 10x24 trunk line coming off the furnace...thats huge for that house.